1. Field of the Invention
This invention relates to a gain control circuit, and more particularly, to a digital gain control circuit with which the gain of an amplifier can be controlled at geometric intervals by digital codes; that is, the gain in logarithmic representation can be controlled linearly in correspondence with digital codes (termed "linearly in decibel").
2. Description of the Prior Art
For example, the loss of a transmission line is given by raising the loss of a unit transmission length to the power of a transmission length. In order to compensate for this transmission loss, therefore, a circuit having characteristics inverse thereto is required. Accordingly, a circuit capable of controlling the gain of the circuit linearly in decibel becomes necessary.
As a circuit arrangement meeting such a requisite, there has heretofore been known one wherein an input side circuit or a feedback circuit of an operational amplifier is provided with a network in which a large number of series (or parallel) circuits each consisting of a switch and a resistance element are connected in parallel (or in series) and wherein any of the switches is selected by a binary code so as to obtain a predetermined gain. Since, however, the prior art circuit arrangement requires a large number of series circuits and adopts the system of once decoding a controlling digital code to convert the same into the code suitable for selecting the switch, it is unsatisfactory for the reduction of the installation cost because of a large number of circuit constituents.